JP4364328B2 - Objective lens for high-density optical recording media - Google Patents

Description

【００２３】
ここで、第２レンズＬ_２は、不遊点の原理を用いたアプラナチックレンズである。すなわち、第２レンズＬ_２は、第１レンズＬ_１による像点と対物レンズ全系による像点とを共役にする。
【００２４】
これにより、第２レンズＬ_２に入射した光束は、その光記録媒体側の面において屈折することなく直進することとなり、無収差かつ正弦条件を満足したものとなる。
【００２５】
さらに、第１レンズＬ_１は、単独で収差補正がなされるように構成されている。
【００２６】
これにより、レンズ全系を設計あるいは評価する際に第２レンズＬ_２のみの収差補正に配慮すればよいこととなる。
【００２７】
また、第２レンズＬ_２における光記録媒体側の面の曲率半径は、対物レンズ全系のバックフォーカス長に略等しい。
【００２８】
また、実施例１に係る高密度光記録媒体用対物レンズは、下記条件式（１）および（２）を満足する。
【００２９】
0.9665 ≦ Ｒ_４／ＢＦ ＜ 1.2 ……（１）
1.45 ＜ Ｆ_１／Ｆ ＜ 1.7 ……（２）
ただし、
ＢＦ：対物レンズ全系のバックフォーカス長
Ｒ_４：第２レンズＬ_２の光記録媒体側の面の曲率半径
Ｆ_１：第１レンズＬ_１の焦点距離
Ｆ ：対物レンズ全系の焦点距離
【００３０】
表１に、実施例１の各レンズ面の曲率半径Ｒ（ｍｍ）、各レンズの軸上面間隔（各レンズの中心厚および各レンズ間の空気間隔）Ｄ（ｍｍ）、各レンズの波長413ｎｍにおける屈折率Ｎ、およびアッベ数νを示す。なお、表１および以下の表において、各記号に対応させた数字は光源側から順次増加するようになっている。
【００３１】
また、表１の中段には実施例１における上記非球面深さ式に示される非球面の各定数の値を示し、下段には実施例１における入射光束径φ（ｍｍ）、レンズ全系の焦点距離Ｆ（ｍｍ）、第１レンズＬ_１の焦点距離Ｆ_１（ｍｍ）、レンズ全系のバックフォーカスＢＦ（ｍｍ）、ならびに条件式（１）および（２）に対応する値を示す。
【００３２】
【表１】

【００３３】
表１に示すように、実施例１は条件式（１）および（２）を満足している。
【００３４】
図２は、実施例１における第１レンズＬ_１の、波長413ｎｍにおける波面収差を表す図である。図２に示すとおり、第１レンズＬ_１は単独でも収差補正が良好なレンズである。
【００３５】
図３は、実施例１のレンズ全系の、波長413ｎｍにおける波面収差を表す図である。図３に示すとおり、この高密度光記録媒体用対物レンズは、収差を良好に補正したレンズであることが明らかである。
【００３６】
以下、上記実施例１の作用について説明する。
【００３７】
第１レンズＬ_１を単玉構成の非球面レンズとし、第１レンズＬ_１単独で収差補正をし、第２レンズＬ_２をアプラナチックレンズとすることで、対物レンズ全系のＮＡ’は以下の式により表される。
【００３８】
ＮＡ’ ＝ ＮＡ_１ × Ｎ_２
ここで、
ＮＡ’：対物レンズ全系のＮＡ
ＮＡ_１：対物レンズの第１レンズＬ_１単独でのＮＡ
Ｎ_２ ：対物レンズの第２レンズＬ_２の屈折率
【００３９】
対物レンズの第１レンズＬ_１に製造実績のあるＮＡ0.6の対物レンズを採用し、第２レンズＬ_２を屈折率が1.5のアプラナチックレンズとすると、対物レンズ全系のＮＡを0.9と高ＮＡのものとすることができる。
【００４０】
ここでアプラナチックレンズについて説明する。
【００４１】
アプラナチックとは球面収差が無く、かつ正弦条件を満足する状態を称する用語であって、１つの球面毎に、この状態を満足する特定の共役点が存在する。
【００４２】
その共役点となるための２条件のうち第１の条件は、第２レンズＬ_２の光源側の面の屈折関係において以下の式が満足されることである。
【００４３】
Ｓ’ ＝ Ｓ ／ Ｎ_２
ここで、
Ｓ’：対物レンズの第２レンズＬ_２の光源側の面における像点距離
Ｓ ：対物レンズの第２レンズＬ_２の光源側の面における物点距離
この第１の条件を満足することにより、第２レンズＬ_２の倍率がその屈折率の逆数となり、対物レンズ全系のＮＡが第１レンズＬ_１単独のＮＡと第２レンズＬ_２の屈折率との積となるとともに、第２レンズＬ_２の光源側の面による球面収差の発生が無く、正弦条件も満足した状態となる。
【００４４】
さらに上記２条件のうち第２の条件は、第２レンズＬ_２の光記録媒体側の面における軸上光線の入射角と射出角が互いに等しい状態となることである。
【００４５】
第２レンズＬ_２の光記録媒体側の面の曲率半径を対物レンズ全系のバックフォーカス長と略等しくすることにより、第２レンズＬ_２の光記録媒体側の面における軸上光線の入射角と射出角を等しくすることができ、第２レンズＬ_２全体にて球面収差の発生が無く、正弦条件も満足した状態となる。
【００４６】
以上の内容は光記録媒体側にカバーガラスが無い光学系を前提としているが、カバーガラスが存在することで、以上に説明した条件が完全に満足されなくとも、以上に説明した内容を適用することで、レンズの設計および製造が容易になる。
【００４７】
また、上記実施例１のレンズでは上述した如く、条件式（１）が満足されている。
【００４８】
すなわち、光記録媒体側にカバーガラスが存在している状態では、第１レンズＬ_１単独で収差を補正しても、第１レンズＬ_１単独のＮＡと対物レンズ全系のＮＡが互いに異なるため、第２レンズＬ_２にアプラナチックレンズを採用しているにも拘わらず、対物レンズ全系で収差が発生することがある。その際には前述した内容を応用して、第２レンズＬ_２を完全なアプラナチックの状態から少しずらすとよい。ただしこの場合には、対物レンズ全系のバックフォーカス長と、対物レンズの第２レンズＬ_２の光記録媒体側の曲率半径がわずかにずれることになるが、上記条件式（１）を満足することにより、対物レンズ全系で収差が補正されたレンズとすることができる。
【００４９】
また、光記録媒体側にカバーガラスが存在している状態では、以上に説明した内容とは逆に、対物レンズ全系での収差を考慮し、第１レンズＬ_１の収差を意識的に発生させるようにしてもよい。
【００５０】
また、上記実施例１のレンズでは上述したように条件式（２）が満足されている。
【００５１】
上記条件式（２）を満足することにより、第１および第２レンズＬ_１、Ｌ_２の仕様を従来のものから大幅に変更することなく製造可能である。
【００５２】
上記条件式（２）の下限を下回った場合、対物レンズ全系を高ＮＡ化しようとすると、第１レンズＬ_１単独のＮＡを高くしなければならず、従来の設計・製造の技術をそのまま適用することが難しくなる。
【００５３】
上記条件式（２）の上限を上回ると、第２レンズＬ_２において焦点距離が短くなる一方で屈折力が大きくなり、第２レンズＬ_２の面の曲率および偏心等の製造誤差が厳しくなり、第２レンズＬ_２の製造コストが上昇する。
【００５４】
＜実施例２＞
図６は、実施例２に係る高密度光記録媒体用対物レンズの構成および光路を示す図である。なお、Ｐは集光点であり、Ｘは光軸を示す。
実施例２に係る高密度光記録媒体用対物レンズは、図６に示すとおり、実施例１と略同様の構成とされ同様の作用効果（全系のＮＡは0.9）を奏するもので、第１レンズＬ_１としては実施例１と同じレンズを用いている。
【００５５】
表２に、実施例２の各レンズ面の曲率半径Ｒ（ｍｍ）、各レンズの軸上面間隔（各レンズの中心厚および各レンズ間の空気間隔）Ｄ（ｍｍ）、各レンズの波長413ｎｍにおける屈折率Ｎ、およびアッベ数νを示す。
【００５６】
また、表２の下段には実施例２における入射光束径φ（ｍｍ）、レンズ全系の焦点距離Ｆ（ｍｍ）、第１レンズＬ_１の焦点距離Ｆ_１（ｍｍ）、レンズ全系のバックフォーカスＢＦ（ｍｍ）、ならびに条件式（１）および（２）に対応する値を示す。第１レンズＬ_１は実施例１と同じレンズを用いているので、その非球面の各定数の値は実施例１と同様である。
【００５７】
【表２】

【００５８】
表２に示すように、実施例２は条件式（１）および（２）を満足している。
図７は、実施例２のレンズ全系の、波長413ｎｍにおける波面収差を表す図である。図７に示すとおり、この高密度光記録媒体用対物レンズは、収差を良好に補正したレンズであることが明らかである。
【００５９】
＜実施例３＞
図８は、実施例３に係る高密度光記録媒体用対物レンズの構成および光路を示す図である。なお、Ｐは集光点であり、Ｘは光軸を示す。
実施例３に係る高密度光記録媒体用対物レンズは、図８に示すとおり、実施例１と略同様の構成とされ同様の作用効果（全系のＮＡは0.9）を奏するもので、第１レンズＬ_１は実施例１と同じレンズを用いている。
【００６０】
表３に、実施例３の各レンズ面の曲率半径Ｒ（ｍｍ）、各レンズの軸上面間隔（各レンズの中心厚および各レンズ間の空気間隔）Ｄ（ｍｍ）、各レンズの波長413ｎｍにおける屈折率Ｎ、およびアッベ数νを示す。
【００６１】
また、表３の下段には実施例３における入射光束径φ（ｍｍ）、レンズ全系の焦点距離Ｆ（ｍｍ）、第１レンズＬ_１の焦点距離Ｆ_１（ｍｍ）、レンズ全系のバックフォーカスＢＦ（ｍｍ）、ならびに条件式（１）および（２）に対応する値を示す。第１レンズＬ_１としては実施例１と同じレンズを用いているので、その非球面の各定数の値は実施例１と同様である。
【００６２】
【表３】

【００６３】
表３に示すように、実施例３は条件式（１）および（２）を満足している。
図９は、実施例３のレンズ全系の、波長413ｎｍにおける波面収差を表す図である。図９に示すとおり、この高密度光記録媒体用対物レンズは、収差を良好に補正したレンズであることが明らかである。
【００６４】
＜実施例４＞
図４は、実施例４に係る高密度光記録媒体用対物レンズの構成および光路を示す図である。なお、Ｐは集光点であり、Ｘは光軸を示す。
【００６５】
実施例４に係る高密度光記録媒体用対物レンズでは、図４に示すとおり、実施例１と略同様の２枚のレンズからなる構成とされ、第２レンズＬ_２の光記録媒体側にカバーガラス１が配されている。
【００６６】
なお、カバーガラス１は、第２レンズＬ_２の光源側の面と集光点Ｐの間の任意の位置に配することが可能である。
【００６７】
本実施例では、光記録媒体側にカバーガラス１が配されていることから完全にアプラナチックな状態とはなっていないが、実施例１と同様に条件式（１）および（２）を満足する。
【００６８】
なお、全系のＮＡは0.9となっている。
【００６９】
表４に、実施例４の各レンズ面の曲率半径Ｒ（ｍｍ）、各レンズの軸上面間隔（各レンズの中心厚および各レンズ間の空気間隔）Ｄ（ｍｍ）、各レンズの波長413ｎｍにおける屈折率Ｎ、およびアッベ数νを示す。
【００７０】
また、表４の中段には実施例４における上記非球面深さ式に示される非球面の各定数の値を示し、下段には実施例４における入射光束径φ（ｍｍ）、レンズ全系の焦点距離Ｆ（ｍｍ）、第１レンズＬ_１の焦点距離Ｆ_１（ｍｍ）、レンズ全系のバックフォーカスＢＦ（ｍｍ）、ならびに条件式（１）および（２）に対応する値を示す。
【００７１】
【表４】

【００７２】
表４に示すように、実施例４は条件式（１）および（２）を満足する値となっている。
【００７３】
図５は、実施例４のレンズ全系の、波長413ｎｍにおける波面収差を表す図である。図５に示すとおり、この高密度光記録媒体用対物レンズは、収差を良好に補正したレンズであることが明らかである。
【００７４】
＜実施例５＞
図１０は、実施例５に係る高密度光記録媒体用対物レンズの構成および光路を示す図である。なお、Ｐは集光点であり、Ｘは光軸を示す。
実施例５に係る高密度光記録媒体用対物レンズは、図１０に示すとおり、実施例４と略同様の構成とされ同様の作用効果（全系のＮＡは0.9）を奏するものである。
【００７５】
なお、実施例５に係る高密度光記録媒体用対物レンズは、条件式（１）を満足するものとなっている。
【００７６】
表５に、実施例５の各レンズ面の曲率半径Ｒ（ｍｍ）、各レンズの軸上面間隔（各レンズの中心厚および各レンズ間の空気間隔）Ｄ（ｍｍ）、各レンズの波長413ｎｍにおける屈折率Ｎ、およびアッベ数νを示す。
【００７７】
また、表５の中段には実施例５における上記非球面深さ式に示される非球面の各定数の値を示し、下段には実施例５における入射光束径φ（ｍｍ）、レンズ全系の焦点距離Ｆ（ｍｍ）、第１レンズＬ_１の焦点距離Ｆ_１（ｍｍ）、レンズ全系のバックフォーカスＢＦ（ｍｍ）、ならびに条件式（１）に対応する値を示す。
【００７８】
【表５】

【００７９】
表５に示すように、実施例５は条件式（１）を満足している。
図１１は、実施例５のレンズ全系の、波長413ｎｍにおける波面収差を表す図である。図１１に示すとおり、この高密度光記録媒体用対物レンズは、収差を良好に補正したレンズであることが明らかである。
【００８０】
なお、本発明の高密度光記録媒体用対物レンズとしては、上記実施例のものに限られるものではなく種々の態様の変更が可能であり、例えば各レンズの曲率半径Ｒおよびレンズ間隔（もしくはレンズ厚）Ｄを適宜変更することが可能である。
【００８１】
また、本発明に係る対物レンズは、上述したように物点（光源）を無限遠方とする場合のみに適用されるものではなく、物点を有限距離に配したいわゆる有限系のレンズとしても使用可能である。
【００８２】
また、本発明の高密度光記録媒体用対物レンズとしては、第２のレンズとしてアプラナチックレンズを用いることが望ましいが、アプラナチックレンズを用いなくとも上記条件式（２）を満足することによりアプラナチックレンズを用いた場合に近い作用効果を得ることができる。
【００８３】
【発明の効果】
以上説明したように、本発明の高密度光記録媒体用対物レンズによれば、２群２枚構成とし、第１のレンズとして従前から単玉光ピックアップ用対物レンズとして知られている両面非球面の両凸レンズを用い、第２のレンズとして不遊点原理を応用した正のメニスカスレンズを用いることにより、レンズの設計、製造、検査の労力およびコストを増大させることなく、かつ収差を良好なものとしつつＮＡ0.6以上の対物レンズとすることができる。
【００８４】
また、本発明の高密度光記録媒体用対物レンズによれば、２群２枚構成とし、第１のレンズとして従前から単玉の光ピックアップ用対物レンズとして知られている両面非球面の両凸レンズを用い、第２のレンズとして正のメニスカスレンズを用い、さらに第１のレンズの焦点距離を所定範囲とすることにより、レンズの設計、製造、検査の労力およびコストを増大させることなく、かつ収差を良好なものとしつつＮＡ0.6以上の対物レンズとすることができる。
【図面の簡単な説明】
【図１】 実施例１に係る高密度光記録媒体用対物レンズの構成および光路を示す図
【図２】 実施例１の第１レンズの波長413ｎｍにおける波面収差図
【図３】 実施例１のレンズ全系の波長413ｎｍにおける波面収差図
【図４】 実施例４に係る高密度光記録媒体用対物レンズの構成および光路を示す図
【図５】 実施例４のレンズ全系の波長413ｎｍにおける波面収差図
【図６】 実施例２に係る高密度光記録媒体用対物レンズの構成および光路を示す図
【図７】 実施例２のレンズ全系の波長413ｎｍにおける波面収差図
【図８】 実施例３に係る高密度光記録媒体用対物レンズの構成および光路を示す図
【図９】 実施例３のレンズ全系の波長413ｎｍにおける波面収差図
【図１０】 実施例５に係る高密度光記録媒体用対物レンズの構成および光路を示す図
【図１１】 実施例５のレンズ全系の波長413ｎｍにおける波面収差図
【符号の説明】
Ｌ_１〜Ｌ_２ レンズ
Ｒ_１〜Ｒ_６ 曲率半径
Ｄ_１〜Ｄ_５ 軸上面間隔
Ｘ 光軸
Ｐ 結像点
１ カバーガラス[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens for a high-density optical recording medium, and more particularly to an optical pickup device for writing or reading information signals on an optical recording medium such as an optical disk, a magneto-optical disk, or an optical card that requires high-density recording. The present invention relates to an objective lens for a high-density optical recording medium used.
[0002]
[Prior art]
In recent years, CD (compact disc), MO (magneto-optical disc), DVD (digital video disc), etc. are often used as optical recording media for recording audio information, video information, and computer data information. It is becoming.
[0003]
In an optical pickup device for writing or reading information signals on these optical recording media, the numerical aperture as an objective lens is about 0.45 for CD, about 0.5 to 0.6 for MO, and 0.6 for DVD. Things are used.
[0004]
In addition, in the optical pickup device, there is a strong demand for downsizing, weight reduction, and cost reduction. In order to satisfy the demand, the objective lens generally adopts an aspherical single lens made of a synthetic resin material or a glass material. It is the target.
[0005]
By the way, in recent years, with the dramatic increase in information recording capacity, the appearance of a recording medium that is as high as possible is eagerly desired, and efforts are being made day and night to improve the recording density of optical recording media.
[0006]
Here, in order to improve the recording density of the optical recording medium, it is necessary to reduce the focused spot diameter by the objective lens. The condensed spot diameter is λ as the wavelength of the light source and NA as the numerical aperture of the objective lens.
k × λ / NA
However, since K is expressed by a constant equation, it is necessary to satisfy at least one of the two conditions of shortening the wavelength of the light source and increasing the NA of the objective lens in order to reduce the focused spot diameter. .
[0007]
Among them, the shortening of the wavelength of the light source has been improved by the shortening of the wavelength of the semiconductor laser and the development of the SHG light source.
[0008]
On the other hand, the condition for increasing the NA of the objective lens has been advanced by adopting a conventional aspherical single lens, and can be achieved by this method until the NA is about 0.6. However, in order to further increase the NA, it is more difficult to manufacture a single lens, and therefore the limit of adopting an aspheric single lens is a problem. For example, the objective lens disclosed in Japanese Patent Laid-Open No. 10-123410 has a two-group, two-lens configuration in order to achieve about NA 0.7.
[0009]
[Problems to be solved by the invention]
However, those described in the above publication are more difficult to design and manufacture than conventional single-lens objective lenses, increasing labor and cost. Furthermore, since the evaluation method is not easy, the labor and cost of product inspection also increase.
[0010]
In view of the above circumstances, the present invention is an objective lens used in an optical pickup device, which has a two-group two-lens configuration with an NA of 0.6 or more, and can be easily designed, manufactured, and inspected, and corrects aberrations satisfactorily. An object of the present invention is to provide an objective lens for a high-density optical recording medium.
[0011]
[Means for Solving the Problems]
An objective lens for a high-density optical recording medium of the present invention is an objective lens for an optical pickup for condensing a light beam on an optical recording medium,
In order from the light source side, a first lens composed of a biconvex lens whose both surfaces are aspherical and a second lens composed of a meniscus lens whose both surfaces are spherical and facing the light source side are arranged,
The radius of curvature of the surface on the optical recording medium side of the second lens is a value that satisfies the following conditional expression (1) .
[0012]
0.9665 ≦ R ₄ / BF <1.2 (1)
here,
BF: Back focus length of the entire objective lens system R ₄ : Radius of curvature of the surface of the second lens on the optical recording medium side
In the objective lens for the high density optical recording medium of the present invention, before Symbol second surface of the optical recording medium side of the lens radius of curvature R ₄ are equal correct it in the back focal length BF of the objective lens system Is desirable.
[0015]
In the objective lens for a high density optical recording medium of the present invention, it is desirable that the numerical aperture of the entire objective lens system is 0.6 or more.
[0017]
Further, in the objective lens for the high density optical recording medium of the present invention, it is desirable to satisfy the lower Symbol conditional expression (2).
[0018]
_{1.45 <F 1 / F <1.7} ...... (2)
here,
F ₁ : Focal length of the first lens F: Focal length of the entire objective lens system
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an objective lens for a high-density optical recording medium according to an embodiment of the present invention will be described using Examples 1 to 5 with reference to the drawings.
[0020]
<Example 1>
FIG. 1 is a diagram illustrating a configuration and an optical path of an objective lens for a high-density optical recording medium according to the first embodiment. Note that P is a condensing point, and X indicates an optical axis.
[0021]
As shown in FIG. 1, the objective lens for a high-density optical recording medium according to Example 1 is a first lens L composed of a biconvex lens in which a surface having a large curvature is directed to the light source side in order from the light source side and both surfaces are aspherical surfaces. _1, formed by arranging the second lens L _2, which is a positive meniscus lens having a convex surface directed toward the light source side is a double-sided spherical. The non-spherical shape of the first lens L ₁ is represented by the depth of an aspheric surface expression shown below.
[0022]
[Expression 1]

[0023]
Here, the second lens L ₂ is A Plana tick lens using the principle of non遊点. That is, the second lens L ₂ is a conjugate of the image point of the first image point by the lens L ₁ and the objective lens system.
[0024]
Thus, a light beam incident on the second lens L ₂ becomes a possible straight without being refracted at the surface of the optical recording medium side, and that satisfies the aplanatic and sine condition.
[0025]
Further, the first lens L ₁ is configured as solely aberration correction is made.
[0026]
Thereby, it is sufficient to consider the aberration correction of the second lens L ₂ only when designing or evaluating the entire lens system.
[0027]
The curvature radius of the surface of the optical recording medium side in the second lens L ₂ is approximately equal to the back focal length of the objective lens system as a whole.
[0028]
In addition, the objective lens for a high-density optical recording medium according to Example 1 satisfies the following conditional expressions (1) and (2).
[0029]
0.9665 ≦ R ₄ / BF <1.2 (1)
_{1.45 <F 1 / F <1.7} ...... (2)
However,
BF: Back focus length of the entire objective lens system R ₄ : Radius of curvature of the surface of the second lens L _{2 on} the optical recording medium side F ₁ : Focal length of the first lens L ₁ F: Focal length of the entire objective lens system ]
Table 1 shows the curvature radius R (mm) of each lens surface of Example 1, the axial top surface spacing (center thickness of each lens and the air spacing between each lens) D (mm), and the wavelength of each lens at 413 nm. A refractive index N and an Abbe number ν are shown. In Table 1 and the following tables, the numbers corresponding to the respective symbols are sequentially increased from the light source side.
[0031]
The middle part of Table 1 shows the values of each constant of the aspheric surface shown in the above aspherical surface depth formula in Example 1, and the lower part shows the incident light beam diameter φ (mm) in Example 1 and the total lens system. The values corresponding to the focal length F (mm), the focal length F ₁ (mm) of the first lens L ₁ , the back focus BF (mm) of the entire lens system, and conditional expressions (1) and (2) are shown.
[0032]
[Table 1]

[0033]
As shown in Table 1, Example 1 satisfies the conditional expressions (1) and (2).
[0034]
2, the first lens L ₁ in Example 1 is a diagram showing the wavefront aberration at a wavelength of 413 nm. As shown in FIG. 2, the first lens L ₁ is a lens that has good aberration correction even when it is used alone.
[0035]
FIG. 3 is a diagram illustrating the wavefront aberration of the entire lens system of Example 1 at a wavelength of 413 nm. As shown in FIG. 3, it is clear that the objective lens for a high-density optical recording medium is a lens in which aberrations are favorably corrected.
[0036]
Hereinafter, the operation of the first embodiment will be described.
[0037]
By making the first lens L ₁ an aspherical lens having a single lens configuration, correcting the aberration with the first lens L ₁ alone, and using the second lens L ₂ as an aplanatic lens, the NA ′ of the entire objective lens system is It is represented by the following formula.
[0038]
NA ′ = NA ₁ × N ₂
here,
NA ': NA of the entire objective lens system
NA ₁ : NA of the first lens L ₁ alone of the objective lens
N ₂ : Refractive index of the second lens L ₂ of the objective lens
If the objective lens of NA 0.6 that has been manufactured is adopted as the first lens L ₁ of the objective lens and the second lens L ₂ is an aplanatic lens having a refractive index of 1.5, the NA of the entire objective lens system is 0.9. It can be of high NA.
[0040]
Here, the aplanatic lens will be described.
[0041]
“Aplanatic” is a term that refers to a state in which there is no spherical aberration and satisfies the sine condition, and for each spherical surface, there is a specific conjugate point that satisfies this state.
[0042]
The first condition of the two conditions for the the conjugate point is that the following expression is satisfied in the refraction relationship surface of the second lens L ₂ of the light source side.
[0043]
S ′ = S / N ₂
here,
S ': distance image point in the plane of the second lens L ₂ of the light source side of the objective lens S: object distance in the light source-side surface of the second lens L ₂ of the objective lens by satisfying the first condition, magnification of the second lens L ₂ is the reciprocal of the refractive index, together with NA of the objective lens system as a whole is the product of the first lens L ₁ alone NA and the refractive index of the second lens L _2, the second lens L _No spherical aberration is generated by the surface of the light source ₂ and the sine condition is also satisfied.
[0044]
Further, the second condition of the two conditions is that the exit angle and the angle of incidence of the axial rays on the surface of the second lens L ₂ of the optical recording medium side becomes equal to each other state.
[0045]
By making the curvature radius of the surface of the second lens L _{2 on} the optical recording medium side substantially equal to the back focus length of the entire objective lens system, the incident angle of the axial ray on the surface of the second lens L _{2 on} the optical recording medium side and can equal the exit angle, occurrence of spherical aberration in the second lens L ₂ whole without a state of sine condition was also satisfactory.
[0046]
The above contents are based on an optical system without a cover glass on the optical recording medium side, but the contents described above are applied even if the conditions described above are not completely satisfied due to the presence of the cover glass. This facilitates lens design and manufacture.
[0047]
In addition, in the lens of Example 1, the conditional expression (1) is satisfied as described above.
[0048]
That is, in a state where the cover glass on the optical recording medium side is present, even if correct aberrations in the first lens L ₁ alone, differ first lens L ₁ alone NA and NA of the objective lens system as a whole to each other , the second lens L ₂ despite adopts the a Plana tick lens, sometimes aberration occurs in the objective lens system as a whole. Its in by applying the aforementioned contents, may slightly shifting the second lens L ₂ from the state of complete Apuranachikku. In this case, however, the back focal length of the objective lens system as a whole, although the radius of curvature of the second lens L ₂ of the optical recording medium side of the objective lens is deviated slightly, to satisfy the conditional expression (1) Thus, a lens in which aberration is corrected in the entire objective lens system can be obtained.
[0049]
Further, in a state where the cover glass on the optical recording medium side is present, contrary to what has been described above, in consideration of the aberration of the objective lens system, consciously generate a first aberration of the lens L ₁ You may make it make it.
[0050]
In addition, the lens of Example 1 satisfies the conditional expression (2) as described above.
[0051]
By satisfying the conditional expression (2), it is possible to manufacture the first and second lenses L ₁ and L ₂ without significantly changing the specifications of the first and second lenses L ₁ and L ₂ .
[0052]
If the lower limit of the condition (2), an attempt to the NA of the objective lens total system, it is necessary to increase the first lens L ₁ alone NA, of conventional design and manufacturing techniques as It becomes difficult to apply.
[0053]
If the upper limit of conditional expression (2) is exceeded, the focal length of the second lens L ₂ is shortened while the refractive power is increased, and manufacturing errors such as curvature and decentration of the surface of the second lens L ₂ become severe, the manufacturing cost of the second lens L ₂ is increased.
[0054]
<Example 2>
FIG. 6 is a diagram illustrating a configuration and an optical path of an objective lens for a high density optical recording medium according to the second embodiment. Note that P is a condensing point, and X indicates an optical axis.
As shown in FIG. 6, the objective lens for a high-density optical recording medium according to Example 2 has substantially the same configuration as that of Example 1, and exhibits the same operational effects (NA of the entire system is 0.9). the lens L ₁ uses the same lens as in example 1.
[0055]
Table 2 shows the curvature radius R (mm) of each lens surface of Example 2, the axial top surface spacing (center thickness of each lens and the air spacing between each lens) D (mm), and the wavelength of each lens at 413 nm. A refractive index N and an Abbe number ν are shown.
[0056]
Further, the lower part of Table 2 the incident beam diameter in Example 2 phi (mm), the lens focal length F of (mm), the focal length _F 1 of the first lens _{L 1} (mm), the entire lens system back The values corresponding to the focus BF (mm) and the conditional expressions (1) and (2) are shown. Since the first lens L ₁ uses the same lens as in the first embodiment, the values of the constants of the aspheric surfaces are the same as those in the first embodiment.
[0057]
[Table 2]

[0058]
As shown in Table 2, Example 2 satisfies the conditional expressions (1) and (2).
FIG. 7 is a diagram illustrating the wavefront aberration of the entire lens system of Example 2 at a wavelength of 413 nm. As shown in FIG. 7, it is clear that the objective lens for a high-density optical recording medium is a lens in which aberrations are favorably corrected.
[0059]
<Example 3>
FIG. 8 is a diagram illustrating a configuration and an optical path of an objective lens for a high density optical recording medium according to the third embodiment. Note that P is a condensing point, and X indicates an optical axis.
As shown in FIG. 8, the objective lens for a high-density optical recording medium according to Example 3 has substantially the same configuration as that of Example 1, and exhibits the same operational effects (NA of the entire system is 0.9). lens L ₁ is using the same lens as in example 1.
[0060]
Table 3 shows the curvature radius R (mm) of each lens surface of Example 3, the axial top surface distance (center thickness of each lens and the air space between each lens) D (mm), and the wavelength of each lens at 413 nm. A refractive index N and an Abbe number ν are shown.
[0061]
Further, the lower part of Table 3 incident beam diameter in Example 3 phi (mm), the lens focal length F of (mm), the focal length _F 1 of the first lens _{L 1} (mm), the entire lens system back The values corresponding to the focus BF (mm) and the conditional expressions (1) and (2) are shown. Since the first lens L ₁ uses the same lens as in Example 1, the values of the constants of the aspherical surface is the same as in Example 1.
[0062]
[Table 3]

[0063]
As shown in Table 3, Example 3 satisfies the conditional expressions (1) and (2).
FIG. 9 is a diagram illustrating the wavefront aberration of the entire lens system of Example 3 at a wavelength of 413 nm. As shown in FIG. 9, it is clear that the objective lens for a high-density optical recording medium is a lens in which aberrations are favorably corrected.
[0064]
<Example 4>
FIG. 4 is a diagram illustrating a configuration and an optical path of an objective lens for a high density optical recording medium according to the fourth embodiment. Note that P is a condensing point, and X indicates an optical axis.
[0065]
The high-density optical recording medium for an objective lens according to Example 4, as shown in FIG. 4, is configured to a substantially same two lenses as in Example 1, a cover on the optical recording medium side of the second lens L ₂ Glass 1 is arranged.
[0066]
The cover glass 1 can be located in any position between the second lens L ₂ of the light source side surface and the focal point P.
[0067]
In this embodiment, since the cover glass 1 is arranged on the optical recording medium side, it is not completely aplanatic. However, the conditional expressions (1) and (2) are satisfied as in the first embodiment. .
[0068]
Note that the NA of the entire system is 0.9.
[0069]
Table 4 shows the curvature radius R (mm) of each lens surface of Example 4, the axial top surface spacing (center thickness of each lens and the air spacing between each lens) D (mm), and the wavelength of each lens at 413 nm. A refractive index N and an Abbe number ν are shown.
[0070]
The middle part of Table 4 shows the values of each constant of the aspheric surface shown in the above aspherical surface depth formula in Example 4, and the lower part shows the incident light beam diameter φ (mm) in Example 4 and the total lens system. The values corresponding to the focal length F (mm), the focal length F ₁ (mm) of the first lens L ₁ , the back focus BF (mm) of the entire lens system, and conditional expressions (1) and (2) are shown.
[0071]
[Table 4]

[0072]
As shown in Table 4, Example 4 has values that satisfy the conditional expressions (1) and (2).
[0073]
FIG. 5 is a diagram illustrating the wavefront aberration of the entire lens system of Example 4 at a wavelength of 413 nm. As shown in FIG. 5, it is clear that the objective lens for a high-density optical recording medium is a lens in which aberrations are favorably corrected.
[0074]
<Example 5>
FIG. 10 is a diagram illustrating a configuration and an optical path of an objective lens for a high density optical recording medium according to the fifth embodiment. Note that P is a condensing point, and X indicates an optical axis.
As shown in FIG. 10, the objective lens for a high-density optical recording medium according to Example 5 has substantially the same configuration as that of Example 4, and exhibits the same operational effects (NA of the entire system is 0.9).
[0075]
The objective lens for a high-density optical recording medium according to Example 5 satisfies the conditional expression (1).
[0076]
Table 5 shows the curvature radius R (mm) of each lens surface of Example 5, the axial top surface distance (center thickness of each lens and the air space between each lens) D (mm), and the wavelength of each lens at 413 nm. A refractive index N and an Abbe number ν are shown.
[0077]
The middle part of Table 5 shows the values of each constant of the aspheric surface shown in the above-mentioned aspheric depth formula in Example 5, and the lower part shows the incident light beam diameter φ (mm) in Example 5 and the total lens system. The values corresponding to the focal length F (mm), the focal length F ₁ (mm) of the first lens L ₁ , the back focus BF (mm) of the entire lens system, and conditional expression (1) are shown.
[0078]
[Table 5]

[0079]
As shown in Table 5, Example 5 satisfies the conditional expression (1).
FIG. 11 is a diagram illustrating the wavefront aberration of the entire lens system of Example 5 at the wavelength of 413 nm. As shown in FIG. 11, it is clear that the objective lens for a high-density optical recording medium is a lens in which aberrations are favorably corrected.
[0080]
The objective lens for a high-density optical recording medium of the present invention is not limited to the above-described embodiments, and various modifications can be made. For example, the radius of curvature R of each lens and the lens interval (or lens) (Thickness) D can be appropriately changed.
[0081]
In addition, the objective lens according to the present invention is not applied only when the object point (light source) is set at infinity as described above, and is also used as a so-called finite lens in which the object points are arranged at a finite distance. Is possible.
[0082]
In the objective lens for a high-density optical recording medium of the present invention, it is desirable to use an aplanatic lens as the second lens, but the above conditional expression (2) is satisfied without using an aplanatic lens. Thus, it is possible to obtain an operational effect close to that when an aplanatic lens is used.
[0083]
【The invention's effect】
As described above, according to the objective lens for a high-density optical recording medium of the present invention, a double-sided aspherical surface that has been known as a single lens optical pickup objective lens as a first lens has a two-group two-element configuration. By using a positive meniscus lens that uses the non-point principle as the second lens, the lens design, manufacturing, inspection labor and cost are not increased, and aberration is improved. And an objective lens having an NA of 0.6 or more.
[0084]
In addition, according to the objective lens for a high-density optical recording medium of the present invention, a double-convex aspherical biconvex lens that has been known as a single lens optical pickup objective lens as a first lens has a two-group two-lens configuration. And using a positive meniscus lens as the second lens, and further setting the focal length of the first lens within a predetermined range, without increasing the labor and cost of lens design, manufacturing, and inspection, and aberrations It is possible to obtain an objective lens with NA of 0.6 or more while improving the lens quality.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration and an optical path of an objective lens for a high-density optical recording medium according to Example 1. FIG. 2 is a wavefront aberration diagram of a first lens of Example 1 at a wavelength of 413 nm. Wavefront aberration diagram of the entire lens system at a wavelength of 413 nm FIG. 4 is a diagram showing the configuration and optical path of an objective lens for a high-density optical recording medium according to Example 4. FIG. 5 shows the wavefront of the entire lens system of Example 4 at a wavelength of 413 nm. Aberration diagram
FIG. 6 is a diagram showing a configuration and an optical path of an objective lens for a high-density optical recording medium according to Example 2.
FIG. 7 is a wavefront aberration diagram of the entire lens system of Example 2 at a wavelength of 413 nm.
FIG. 8 is a diagram showing the configuration and optical path of an objective lens for a high-density optical recording medium according to Example 3.
FIG. 9 is a wavefront aberration diagram of the entire lens system of Example 3 at a wavelength of 413 nm.
FIG. 10 is a diagram illustrating a configuration and an optical path of an objective lens for a high-density optical recording medium according to Example 5.
FIG. 11 is a wavefront aberration diagram of the entire lens system of Example 5 at a wavelength of 413 nm.
L ₁ ~L ₂ lens R ₁ to R ₆ of curvature radius D ₁ to D ₅ axial distance X Optical axis P Imaging point 1 cover glass

Claims (4)

An objective lens for an optical pickup for condensing a light beam on an optical recording medium,
In order from the light source side, a first lens composed of a biconvex lens whose both surfaces are aspherical surfaces and a second lens composed of a meniscus lens whose both surfaces are spherical surfaces and whose convex surfaces are directed to the light source side are arranged. In an objective lens for a recording medium,
The objective lens for a high-density optical recording medium, wherein the radius of curvature of the surface on the optical recording medium side in the second lens is a value satisfying the following conditional expression (1).
0.9665 ≦ R ₄ / BF <1.2 (1)
here,
BF: Back focus length of the entire objective lens system R ₄ : Curvature radius of the surface on the optical recording medium side of the second lens 2. The objective lens for a high-density optical recording medium according to claim 1, wherein a radius of curvature R ₄ of a surface of the second lens on the optical recording medium side is equal to a back focus length BF of the entire objective lens system.   3. The objective lens for a high-density optical recording medium according to claim 1, wherein the numerical aperture of the entire objective lens system is 0.6 or more. The objective lens for a high-density optical recording medium according to claim 1, wherein the following conditional expression (2) is satisfied.
_{1.45 <F 1 / F <1.7} ...... (2)
here,
F ₁ : Focal length of the first lens F: Focal length of the entire objective lens system

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